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Can Sockeye Salmon Outswim Climate Change?

Author: Dahlia Shammas

INTRODUCTION

Sockeye salmon, also known as Oncorhynchus nerka, are very unique animals. They have a distinctive and prehistoric look, as well as a unique lifestyle. They are well known for their strenuous and dangerous journey back to their home stream to spawn (lay and fertilize eggs). This journey is hard enough for sockeye salmon because of predation and how much energy it takes to travel upstream to spawn. Now, anthropogenic (human-caused) climate change has presented another challenge for sockeye during migration. Climate change has led to increased river temperatures in the summer during salmon migration, changing their behavior, physiology, and mortality rates.(1)

Sockeye salmon play an important role in the lives of many species, including ours. They are economically, culturally, and ecologically important. They are one of the most commercially fished species in British Columbia, Canada, and Alaska, United States.(2) They are also culturally very important to the St'át'imc people, the original inhabitants of Fraser River in British Columbia. Sadly, climate change has had negative impacts on how they traditionally fish and preserve sockeye.(3) Ecologically, they are a highly nutritious food source for many animals including bears, birds, wolves, sharks, fish, and other marine mammals. Some species like bears rely on sockeye salmon as a crucial source of energy. Additionally, their carcasses provide nutrition to animals and valuable nutrients such as nitrogen and phosphorus to the environment.

Sockeye salmon are found along the west coast of North America in the Northern Pacific Ocean. This blog post will focus mainly on sockeye salmon that inhabit Fraser River in British Columbia, Canada. Both male and female sockeye salmon look similar in size and color. Males are slightly brighter in color and have a large dorsal hump as can be seen in the photo above. Both sexes range in size from 20 to 28 inches.(4) Most sockeye salmon are anadromous, meaning they are born in freshwater rivers or lakes, travel to the ocean where they spend most of their adult life, and then travel back to their home stream to spawn. In freshwater, their diet consists of zooplankton, amphipods (shrimp-like creatures), and insects.(5) After migrating to the ocean they continue to feed on zooplankton but also eat fish and the occasional squid.(5) After birth, they mature in freshwater for one to three years before migrating to the Pacific ocean. After two to three years in the ocean, they migrate back to their home stream to spawn.(5) Most sockeye start this migration in June and July. Their spawning time varies depending on the population and can take place anywhere between June and December.(5) When they reach their home stream, females dig holes known as redds with their tails and deposit between 2,000 to 4,500 eggs.(5) Males often compete to fertilize the eggs and the females then cover them up with gravel to protect them from the elements and potential predators. After spawning only once, the salmon have fulfilled their lifetime reproductive duty and usually die within weeks.

This blog post will elaborate on how climate change is affecting sockeye salmon behavior, physiology, and mortality. Warmer river temperatures have advanced the timing of their migration to spawn by at least two weeks.(1) Warmer waters have also led to physiological adaptations at the population level for these harsher conditions as well as cardiac collapse when temperatures get too high.(6) A rise in parasitic and fungal infections is also seen in higher water temperatures which could lead to decreased fitness and death.(1) Increased temperatures are also correlated with higher mortality rates of adult salmon as they migrate upstream back to their spawning grounds.(6)

 

BEHAVIORAL CHANGE

Some sockeye salmon populations are migrating earlier to their spawning grounds because of warmer river temperatures in the summer caused by climate change.(1) Historically, populations of salmon start the migration to spawn in the summer around the same time, rarely changing that timing by more than a week.(1) Many populations exhibit a holding behavior, where they rest for 2-6 weeks in the Strait of Georgia (close to the Fraser River) before continuing their journey upstream.(1) Since 1995, this holding behavior has been abandoned by many populations of salmon.(1) Instead, these populations of sockeye advance their migration upstream by 2-8 weeks, significantly increasing the amount of time spent in freshwater compared to historical populations.(1) This is because their population-specific spawning dates do not change even though their migration time has advanced by at least two weeks.(1) This behavioral change in migration is cause for concern because early river entry has been associated with high levels of mortality ranging from 60%-90%.(1)

 

PHYSIOLOGICAL ADAPTATION

Rising river temperatures have had significant impacts on the physiology of sockeye salmon. Salmon burn energy quicker, could have increased stress levels, and possibly die if the water temperature gets too high. Some sockeye populations have developed physiological adaptations that allow them to better survive in higher temperatures. These cardiorespiratory adaptations include greater aerobic scope, increased heart size, and improved coronary supply.(6) A greater aerobic scope is defined as the increased capacity of oxygen consumption of an animal during exercise. Coronary supply is the rate at which oxygenated blood is pumped to the heart. These cardiorespiratory adaptations are different among populations in the Fraser River.(6) Each population of sockeye salmon has a different tolerance to temperature. This physiological limitation is possibly due to cardiac collapse caused by increased temperatures.(6)

 

SOCKEYE SALMON MORTALITY AND LOWERED FITNESS

Climate change has significantly raised the mortality rates of sockeye salmon during migration. As previously mentioned, warmer river temperatures have caused some salmon to prematurely migrate upstream to spawn. Early-entry salmon populations have elevated mortality rates from 60%-90% as a direct cause of historically higher migratory temperatures.(4) Exposure to higher river temperatures can result in the quick depletion of energy reserves that salmon rely on when they stop feeding during migration.(1) These reserves are crucial for spawning and if they are used too early exhausted salmon will not successfully reproduce, lowering fitness (the ability to produce viable offspring).(1)

Warmer water temperatures because of climate change also promote parasitic and external fungal infections in sockeye salmon.(1) An example being the parasite Parvicapsula minibicornis, which is found in the kidney.(1) This paper states that the infection rate of this parasite dramatically rose in sockeye salmon populations that experienced an increase in river temperature. It was also noted that warm-treated salmon (18°C) had more external fungal infections than cold-treated salmon (10°C).(1) A rise in parasitic and fungal infections can lead to increased mortality rates among sockeye salmon. The mechanisms by which they do this are not yet clear, but it is shown that they lead to increased metabolic stress and overuse of energy stores. This lowers overall fitness and could ultimately lead to death.

 

CONCLUSION

Climate change has increased temperatures in rivers where sockeye migrate, causing behavioral change, physiological adaptation, and increased mortality rates at the population level. Warmer water temperatures can advance the river entry time of salmon populations, increasing mortality rates by 60%-90%. Some sockeye populations have developed physiological adaptations that improve cardiorespiratory performance in warmer river conditions. Additionally, high water temperatures can lead to death because of cardiac collapse. Rising temperatures also increase parasitic and fungal infections, which can lead to decreased fitness and increased mortality rates. Exposure to warmer waters lowers the overall chance of survival and reproduction of some sockeye salmon populations. This is particularly concerning for salmon because they only reproduce once in their lifetime. If their health is threatened on their journey to spawn they might not ever successfully reproduce.

Some efforts are being made to conserve sockeye salmon populations. An article by The Nature Conservancy stated that in Alaska, streams are being restored from damage caused by logging. Alaskan rainforests are also being restored by the Hoonah Native Forest Partnership.(7) This is beneficial to salmon because they need strong tree roots in river banks for stability, shade, and protection. There are also multiple organizations educating the public about sockeye salmon and how humans threaten them.(7) Researchers at the Eagle Fish Hatchery of the Idaho Department of Fish and Game are also working hard to preserve sockeye salmon populations. Their goal is to change from a hatchery-based recovery system to one that relies more on salmon reproduction in the wild. They want to ultimately avoid the extinction of sockeye salmon and support sport and tribal harvest needs. More about their efforts can be read here.

A lot is still unknown about how climate change affects sockeye salmon. The environmental factors that influence migration should be further explored. More research should be done to see if sockeye salmon as a species are changing their migratory behavior in a beneficial or harmful way. How parasitic and fungal infections increase mortality rates in sockeye salmon should be investigated. Specifically, the impacts of the parasite Parvicapsula minibicornis on energy use and kidney function. Additionally, it is very important to determine whether sockeye salmon can physiologically and behaviorally adapt in time in order to survive and eventually thrive in increasing river temperatures. Hopefully, through continued research and conservation efforts, sockeye salmon will be able to adapt and “outswim” the negative effects of climate change.


Citations

  1. Crossin GTCT, Hinch SGHG, Cooke SJCJ, Welch DWWW, Patterson DAPA, Jones SRMJRM, et al. Exposure to high temperature influences the behaviour, physiology, and survival of sockeye salmon during spawning migration. Can J Zool [Internet]. 2008 Feb 18 [cited 2021 Apr 23]; Available from: https://cdnsciencepub-com.oca.ucsc.edu/doi/abs/10.1139/Z07-122
  2. Sockeye Salmon [Internet]. Oceana. [cited 2021 Apr 23]. Available from: https://oceana.org/marine-life/ocean-fishes/sockeye-salmon
  3. Jacob C, McDaniels T, Hinch S. Indigenous culture and adaptation to climate change: sockeye salmon and the St’át’imc people. Mitig Adapt Strateg Glob Change. 2010 Dec 1;15(8):859–76.
  4. Sockeye salmon identification - King County [Internet]. [cited 2021 Apr 23]. Available from: https://kingcounty.gov/services/environment/animals-and-plants/salmon-and-trout/identification/sockeye.aspx
  5. Fisheries N. Sockeye Salmon | NOAA Fisheries [Internet]. NOAA. 2021 [cited 2021 Apr 23]. Available from: https://www.fisheries.noaa.gov/species/sockeye-salmon
  6. Eliason EJ, Clark TD, Hague MJ, Hanson LM, Gallagher ZS, Jeffries KM, et al. Differences in Thermal Tolerance Among Sockeye Salmon Populations. Science. 2011 Apr 1;332(6025):109–12.
  7. 5 Ways People Are Helping Wild Salmon [Internet]. [cited 2021 May 7]. Available from: https://www.nature.org/en-us/about-us/where-we-work/united-states/alaska/stories-in-alaska/alaska-five-ways-we-help-salmon/

Image Credit

Two sockeye salmon swimming in different directions” by Milton Love, Marine Science Institute, University of California, Santa Barbara, CA 93106. Obtained from Wikipedia https://en.wikipedia.org/wiki/Sockeye_salmon#/media/File:Sockeye_salmon_swimming_right.jpg.This photograph has been released into the public domain by its author, Milton Love and is available under the Creative Commons CC0 License.

"Fraser River, Canada, near the "West Fraser Historic Trail" (north of Lillooet)” by China Crisis. Obtained from Wikipedia. This photograph is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.

 

 

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